An Equalizer Circuit for Bass and Treble

  1. An equalizer (or EQ) is a circuit used to adjust the frequency response of an input audio
    signal. Develop an equalizer circuit that takes an audio output (for instance, from a Fender
    Stratocaster electric guitar) and provides manually-controlled EQ (boost or attenuation) to
    adjust the amplitude of two channels: bass (20 1000 Hz) and treble (1 kHz 20 kHz).
    The output of the EQ circuit must provide sucient amplication to supply 125mW to a
    As with the previous project, you are restricted to using discrete transistors, diodes, and
    other passive lumped components. You may not use packaged integrated circuits (such as
    logic gates or operational ampliers).
    To complete this project you are expected to develop a plan of attack with your lab partners
    and to do the necessary background research required to successfully complete the work.

    What my lab partners and I did

    We do no know the impedence of the guitar my professor is going to use. We decided that a
    buffer (an emitter follower) would be best to take care of that. We tried to get 1mA across the
    emiiter, and I think we have it but I am not sure.
    The next element in our design is a James Baxandall passive tone control network. We actually have one that works but not for the values that are specified in the lab document. (Normally, my two partners do analysis and I build. They attempt to build and I attempt analysis, but I am not strong with analysis and they never build until they do analysis. Thi causes us to have circuits that work but not for the values we should have. I litterally just pick random resistors and capacitors out of my shoebox and put them together. I learned a lot that way and have some interesting lab sessions.)
    The tone control network is followed by the amplifier (a common emitter). There is real analysis for this part. We have a gain of 2 but we need a gain of 10.
    The amplifier then goes to a push pull which goes to the speaker.

    All the work that we have done is in the attachment withichis soon to follow.
    *The analysis we did said to use a 6k resistor but we got some clipping. We chose random resistors until we got one with no clipping (we used the 1.8k, which the resistor we used to give us the gain of 2)

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    Last edited: Apr 8, 2009
  2. jcsd
  3. more attachments (sorry, but I do not want to go against the terms of service and not show you my efforts in finding the correct implementation).

    Attached Files:

    Last edited by a moderator: Apr 8, 2009
  4. berkeman

    Staff: Mentor

    Welocome to the PF, 7emeraldempre. Are you looking for advice on any particular aspect of your lab project? I'd recommend starting with a clear block diagram of your equalizer, and then showing the circuit in each block (kind of like the pages you posted above).

    Also, you shouldn't be chosing bias and gain resistors at "random". You should strive to understand how to calculate the best values for each type of circuit, given the parameters you are working with (like frequency ranges).

    Also, the output push-pull stage is a bit simple -- what could be added to increase fidelity (at the expense of a little more power dissipation in that stage)?
  5. I know, it is really bad engineering to do that, there's really no excuse if I want to become an electrical engineer. Right now I am a second year student and this is only my second class toward my major. I really do not know how to answer you question about what I could add.
    I guess I would be asking for someone to point out the holes in my project and if there are better ways to do this. I am not familiar with parts beyond resistors, capicitors, inductors, transistors, and diodes.
    In reference for the tone control, based on my understanding, for bass cut, my corner frequencies should be 20 to 500 hertz, bass boost 500 to 1000 Hz, 1000 to 10KHz for treble boost and 10KHz to 20KHz for treble cut? Am I right?
    Decibels are measures of loudness, so I think that I shoul design the bass range to like maybe 30 dB, and treble to be 30 dB also. Is that correct thinking?
  6. berkeman

    Staff: Mentor

    Have you learned about Bode plots and frequency response yet? That's a good way to be thinking about the bass-mid-treble parts of your signal range that you are amplifying.

    I'm not sure I understand the terms you are using "cut" and "corner", but if you are talking about two bandpass filters, one for bass and one for treble, then your corner frequencies match the problem statement. It would still be best to draw a block diagram of your system first, and then go into the details of each block.

    Do you have any SPICE simulation software to use as part of this class? There are free packages available online. It would be good for you to be simulating your circuits as you are designing them.
  7. Hello. I have been having a busy Easter Week and that is why I have not been posting though I am very grateful for your insight. I do not have any SPICE program though (I find I can neverfind one is straight forward enough to use) and we have learned BODE plots. Is this what you were talking about when you said to have a block diagram (the attachemnt)? If so, I did have one in my lab book, I just did not scan in the whole lab entry for this the whole lab entry for this project.

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    Last edited: Apr 12, 2009
  8. berkeman

    Staff: Mentor

    That's getting closer. The drawing is a little hard to read (do you have Visio?), but it looks to me like your treble and bass adjustments are not independent. Are they in series? You want an equalizer's adjustments to be independent. That is, adjusting the bass control should not change the treble passband, and visa-versa. How can you change your circuit for that block to acheive that?
  9. No,I do not have visio, but I will try to di better for the scans. Since I have them seperated will I now have to put them in parallel to the emitter follower and the common emitter, or do i have to build those seperately, one follower and one common emitter for each? I there a SPICE program that you recommend to use?

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